1. Field of the Invention
The present invention concerns a method for examination subject-dependent determination of parameters to control the gradient coils and radio-frequency coils of a coil array of a magnetic resonance device to generate a radio-frequency pulse with which the nuclear spins in the examination region of the examination subject can be moved out of the rest state by an arbitrary angle.
2. Description of the Prior Art
It is known to vary the amplitude and phase of the current of a radio-frequency coil in a magnetic resonance apparatus in a time-dependent manner in order to move the nuclear spins in the examination region of an examination subject out of their rest position. This time-dependent series of amplitudes and phases is also called an excitation profile. These pulses—known as sinc pulses—in combination with a constant gradient field produce the excitation of a slice with a rectangular cross section profile. If the excitation profile has the shape of a Gaussian distribution, the excited slice also exhibits a Gaussian profile.
The relationships between the excitation profile of the radio-frequency pulse and the cross sectional shape of the excited slice is provided for small pulse angles by the Fourier transformed excitation profile of the radio-frequency pulse. In this context, “small” indicates an angle of approximately ≦5°.
Such a simple calculation of an excitation profile is only possible under the assumption that the sensitivity of the radio-frequency coil or of the radio-frequency coils is homogeneous in the entire examination region. Otherwise, the spins in different regions of the excited slice are deflected with different angles.
In reality, the assumptions that are made are rarely fulfilled. Excitation angles of 90° are always used in spin echo-based sequences and a homogeneous coil sensitivity is never present in modern coil arrays. Otherwise the signals acquired by means of parallel imaging cannot be differentiated, which would make parallel imaging impossible.
Moreover, it is not sufficient to take into account only the coil sensitivities of individual coils of a coil array in a homogeneous phantom since the deflection angles generated by the coils also depend on the condition of the examination subject. The excitation profiles of the individual coils must accordingly be recalculated for each examination subject.